1. Field of the Invention
The present invention relates to an apparatus and method for analyzing conditions along a length of an elongate electrically conductive member, and more particularly to such an apparatus and method which is particularly adapted to detect the presence and location of anomalies along the length of a pipeline, such as an oil or gas pipeline that is buried underground or which extends along a floor of a body of water.
2. Background Art
Pipelines which carry oil or some other fluid are often placed underground and extend for possibly hundreds of miles. Such pipelines are commonly made of metal (e.g., steel) and are wrapped with a protective layer of tape to prevent corrosion of the metal. Even so, the protective layer will sometimes deteriorate at certain locations, or possibly be abraded by some object (e.g., a rock which might come in contact with the protective layer) so as to expose the metal of the pipe to the adjacent ground, resulting in premature pipe corrosion.
In order to alleviate this corrosion of the pipeline, it is common to utilize a source of electrical direct current power to impart a negative charge to the pipeline relative to the adjacent ground. One method is to attach Galvanic anodes to the pipe (e.g. a magnesium anode). Another method is to provide a DC generator with the negative output being attached to the pipeline, while the positive output is connected to an electrode which is placed in the ground. However, this also has its shortcomings. For example, there can be a localized interfering electrical field which may reverse the electrical potential between the pipeline and the ground within an area. This electrical field could result, for example, from an adjacent pipeline which might cross (or extend adjacent to) another pipeline.
Accordingly, the pipeline industry has undertaken to analyze the conditions along the length of the various pipelines to determine the electrical potential between the pipeline and the adjacent ground. The common method of doing this is what is termed the "half-cell" process, which has more or less become the standard of the industry. A typical half-cell comprises a containing member which is a sealed plastic cylinder with a porous ceramic plug. A solution of copper sulphate is in the container and there is a piece of copper which extends into the solution of copper sulfate, with this copper being in turn attached to a wire which is then attached to a volt meter. The other lead of the volt meter would lead to a connection to the actual pipe itself. A somewhat crude method of taking half-cell readings would be to walk along the length of the pipe, dig a hole at selected locations to expose the pipe, attach one electrode to the pipe, and then stick the half-cell in the ground at that location to take a reading. Then the person would proceed to the next location along the pipeline and repeat the same process. However, there are more effective methods of accomplishing this. One method is to connect one end of a cable to the pipe at one location, and have the length of the cable wound on a rotating drum which is in turn mounted to a truck. The truck is then driven down the length of the pipeline for a few miles, with the half-cell being placed in the ground at various locations along the length of the pipeline.
When one realizes that pipelines extend beneath freeways, underneath rivers, underneath the ocean floor, and through other areas of difficult access, it can be seen that there are practical problems in employing the half-cell method. Nevertheless, the half-cell method has in a sense become the standard of the industry, and substantial work has been done in analyzing the data gathered through the half-cell method and correlating this to the condition of pipelines in the soil. The net effect is that there has been for many years a growing problem of substantial magnitude in effective detection of pipeline defects. In the United States alone, there is a vast network of pipelines extending along various routes, and there are conferences held between the various owners/operators of such pipelines to resolve the problems associated with these pipelines (e.g., the electrical field of one pipeline affecting another pipeline adversely). Also, the increasing sensitivity to environmental considerations associated with pipeline leaks is of greater concern. Further, the economic considerations of proper maintenance and functioning of these pipelines is significant.
Another prior art method of analyzing the condition of various objects is time domain reflectometry, where a pulse is transmitted along the length of the member being tested, and at the location of a discontinuity, there is a reflection of the pulse which is sent back to a receiving location (which can be the location at which the pulse was transmitted). By measuring the time increment from the transmission of the pulse to the time the reflection is received, while knowing the velocity of the pulse, the location of the discontinuity can be ascertained. Also, depending on the circumstances, the character of the reflected pulse may yield information about the nature of the discontinuity. While this method has value for certain applications, to the best knowledge of the applicant, this has not proven to be an effective method of analyzing the conditions of pipelines.
A search of the patent literature has disclosed a number of U.S. patents these being the following.
U.S. Pat. No. 4,755,742 (Agoston et al) describes a dual channel time domain reflectometer used to avoid multiple reflections of the test pulse.
U.S. Pat. No. 4,739,276 (Graube) also shows a time domain reflectometer which permits the examination of the magnitude of impedance faults along a cable.
U.S. Pat. No. 4,538,103 (Cappon) shows a time domain reflectometer for testing a cable in conjunction with an oscilloscope or other display apparatus. Positive and negative pulses are generated, and these are synchronized with one another. Both pulses are generated at one end of the cable by the same piece of equipment.
U.S. Pat. No. 4,291,204 (Crick) shows a system which can be used with a time domain reflectometer system. An arc is created to provide a low resistance from which the time domain reflectometry signal is reflected.
U.S. Pat. No. 4,289,019 (Claytor) shows a system for detecting leaks in buried pipes. There is provided a plurality of acoustic detectors associated with the pipe under test. The signals reaching the various pickups are compared and a location of the leak can be determined. In a second embodiment shown in FIG. 4, there are two pickups placed at the same section of the pipe, one to sense longitudinal waves and the other torsional waves. The two outputs are compared and a statistical analysis can be obtained as to the location of the leaks.
U.S. Pat. No. 3,600,674 (Roberts et al.) shows a system in which there is a data carrying cable buried below the pipeline that is to be tested. If the material carried by the pipe leaks, it affects the cable so as to produce conduction discontinuity anomalies in the cable. The discontinuities are in turn measured with a time domain reflectometer to determine the location of the leak. This requires burying the cable with the pipeline.
U.S. Pat. No. 2,887,652 (Bendayan et al.) discloses a system utilizing the principle of time domain reflectometry, this showing some of the earlier work in such systems.
U.S. Pat. No. 2,725,526 (Stringfield et al.) provides the means for determining a line fault by recording the initial surge that starts the fault and the reflected signals. By timing the arrival of various parts of the initial signal and the reflected signals, the location of the fault may be determined.
U.S. Pat. No. 2,602,834 (Leslie et al.) shows a device for locating faults in transmission lines, utilizing reflected wave technology for fault detection. There is a source of RF energy that is coupled to the line under test and a receiving system. The RF pulse is generated and sent down the line and if there is a fault in the line, a signal is reflected back. The nature and timing of the reflected signal is an indication of the fault and its location. This again depends upon the reflected signal to determine the fault.
U.S. Pat. No. 4,118,662 (Weber) shows a system for locating underground structures such as pipelines by the injection of a signal that is detected by equipment carried by an operator.
U.S. Pat. No. 4,063,161 (Pardis) shows a means for detection of faults in a cable by propagating a pulse into the cable and detecting the leakage point by profiling the ground potentials. Thus, it would not be possible to utilize this system effectively where the location of the fault is in an inaccessible area.
U.S. Pat. No. 3,924,179 (Dozier) discloses a means of finding a single "dead" wire in a cable bundle by observing the effect of a DC pulse being passed through the cable and detecting the generated field with a receiving instrument.
U.S. Pat. No. 2,113,749 (Statham) discloses a geophysical prospecting system where a series of signals are generated into the earth, and the propagation of the signals through the geophysical formations is determined.